Conventional ground drilling equipments or techniques normally drill holes with a single or outer diameter (OD) target. Depending on the ground condition, a casing sometimes is inserted in the ground to prevent collapse of soil when the ground condition is loose. In the case where the ground strata are stable, no casing is necessary. In either case, all the materials inside the hole will needed to be excavated away in the process of drilling.
One design of drilling equipment has a single down-the-hole (DTH) percussion hammer. With this design, the hole drilled has a maximum possible size of about 1500 mm in diameter. Such single DTH hammer drilling will result in complete excavation of all materials inside the hole drilled as well.
Another design of drilling equipment has a number of DTH percussion hammers arranged over a circular disc and housed inside a circular housing. Because all the materials of the circular hole drilled will be fully excavated, this design is limited only to drilling solid circular holes, not annulus ring shaped holes.
It is also common to use a rotary drilling machine to drill holes with auger in relatively soft ground or core barrel in harder ground condition. This kind of drilling method will also entail the excavation of all materials inside the hole leaving only a solid circular hole.
There is also another drilling technique that utilizes a casing oscillator to first drive a casing down into the ground by oscillation and pushing. In order to continue driving the casing down into the ground, it is necessary to excavate the materials inside the cased hole during the drive by tools such as hammer grabs and chisels. As such, this drilling technique cannot be used to form annulus ring hole. The penetration speed is also very low.